The present invention relates to a glass container forming mold, more particularly to a glass container forming mold which is provided with an inner member formed of a carbon material.
A glass container such as a narrow neck glass bottle and a wide neck glass bottle, is usually made by forming a parison from a gob of molten glass at about 1100.degree. to 1200.degree. C. by blowing or pressing at a parison forming station of an IS machine provided with a blank mold and a neck ring, transferring the hot parison to a final blowing station provided with a blow mold and a bottom plate and then blowing the parison into the container. The blank mold, the neck ring, the blow mold and the bottom plate are conventionally made of a metal, usually of cast iron. In this specification the term of the mold is referred to including the neck ring and the bottom plate and the like.
It is necessary to coat the inner surfaces of the blank mold, the blow mold, the neck ring and the bottom plate with a lubricating and releasing agent so as to prevent defects such as wrinkles from generating on the surface of the product by decreasing friction resistance between the inner surface and the molten glass during forming, and facilitate removing the parison and the finished product from the molds and the like.
As the lubricating and releasing agent for this purpose, for example, a dope comprising graphite powder dispersed in an oily liquid, is often used. However, the graphite powder is lost rapidly by oxidation or attaching to the parison or the final product owing to high temperatures subjected to during forming, and one coating of the dope can withstand only about 50 to 100 forming operations or, for example, about 20 minute operation. This results in decrease of production rate, needs labor of operators for coating and further tends to cause dirty products.
Further, microcracks often generate on the surface of the parison even when the lubricating and releasing agent is used in the step of parison forming, and cause poor final products since they remain on the container formed by blowing at the final blowing station.
As a countermeasure it is thought to employ a combination forming mold consisting of, for example, an outer member of metal, e.g. cast iron and an inner member of a carbon material, since a forming mold consisting of only carbon material may be entirely unpractical because it tends to rupture. However, in this case it is practically difficult to machine both the members to allow the inner surface of the outer member and the outer surface of the inner member to be brought into tightly close contact with each other, and usually a gap of more than 10 .mu.m is formed between the both.
Heat insulating air film is formed in the gap and hinders heat from dissipating from the inner member. Further, the amount of the gap is not uniform, for example, is different circumferentially. As a result the temperature distribution on the inner surface of the inner carbon member becomes uneven, and local burning on the inner surface of the inner carbon member tends to occur. The local burning causes surface defects on the parison and the final product.